A typical prior art postage meter (franking machine) system has a powerful AC motor which drives all of the moving parts of the meter when a letter or other mail piece enters the paper path. The postage meter is, by definition, prepared at all times to be able to rotate so as to print postage. If it is not ready to print postage, it blocks actuation of the AC motor.
Generally the system includes a base which contains the motor or motors and contains the lower portion of the paper path, for example a number of rollers or belts (or both) and provides a level and smooth surface defining part of the paper path. The rollers or belts protrude slightly above the level, smooth surface.
Above the base, and removably mounted to the base, is the postage meter proper. The postage meter typically has a secure housing within which are found accounting registers indicative of the postage value available for printing, and a print mechanism such as a print rotor. The meter typically provides additional rollers which, together with rollers in the base, define part of the paper path.
A mechanical linkage such as a gear engagement links the upper part of the paper path (within the postage meter) with the lower part of the paper path (within the base) and with the print mechanism such as the print rotor.
It should be appreciated that while some of the upper rollers or belts that help to define the paper path may be within the meter, others of the upper rollers or belts may be provided by part of the base. Thus, as a mail piece progresses along a level paper path, it may initially have the base both above it and below it, while later it may have the meter above it and the base below it.
In such prior-art systems, as mentioned above, it is commonplace to link the moving parts so that they are all powered by a single large AC motor. A single-revolution clutch may be provided which is actuated once for each time that a mail piece arrives at the postage meter. For example, there may be a trigger in the paper path just prior to the print rotor, and when the trigger is actuated then the single-revolution clutch causes the print rotor to rotate once.
Such a system has the advantages of simplicity and long-standing use in the field. But the AC motor is heavy and consumes a lot of power, and is actually rather crude in terms of speed control. Typically the entire system is forced to start at rest, accelerate to high speed, and then decelerate back to rest. The cycle is repeated hundreds of thousands or millions of times, and the extreme nature of the cycle requires the drive train to be quite strong and heavy. The cycle is also noisy. The AC motor is typically heavy and bulky.
Another prior art approach for a postage meter is that described, for example, in U.S. Pat. No. 4,631,681 or 4,774,446. In such a system, two motors are employed, the first an AC motor that actuates the paper path, and a second motor which is a DC motor having encoder feedback, coupled with the print rotor, driven with pulse-width modulation to match the velocity of the paper path driven by the AC motor, as measured with two document sensors in the paper path as driven by the AC motor. Such a system offers some advantages over the prior art, but at the expense of requiring more than one document sensor in the paper path. Such a system also has the possible drawback that it may in fact fail to measure the velocity accurately and may drive the print rotor at a speed that is too fast or too slow relative to the speed within the AC-driven paper path.
Still another prior art approach is that described in U.S. Pat. No. 4,933,616, in which a microcomputer provides control of the speed of first and second motors, a first motor connected with a sheet transport system and a second motor connected with a print drum assembly. Such a system has the drawback that both of the motors must be of a type that can have closely controlled speed, and each motor must have driving circuitry associated with the speed control capability. This adds to the cost of the system and to its weight and complexity.
It is desirable to have a postage meter system that overcomes the drawbacks of the prior art, in which the print rotor is driven in faithful correspondence with the paper path, and in which the operation is quieter, smoother, less expensive, and less bulky than in the prior art.